2 experts and 1 parent have answered
First, I should say that your attorney is the best person to answer this question. Document any evidence of these occurrences and pass them on to your lawyer/solicitor immediately.
There are so many methods for the non-custodial parent to contact a child these days. Before trying to discover all of these, talk to your daughter about the consequences of her dad violating any court-ordered visitation practices. Many non-custodial parents aren't aware that emails, phone conversations, and texts may be part of any visitation order even if they are not mentioned specifically.
For example, my daughter's mom cannot (by court order) have unsupervised visitations with her. The "spirit" of the court order means that phone calls must also be supervised even though the court order doesn't specifically state this. It is up to the custodial parent to understand these complex gray areas of the law. It's exhausting but important.
When your daughter understands that her dad, whom I'm sure she loves dearly, could get into trouble with the court, she may be more inclined to share with you when dad tries to contact her covertly. The other option is to try to follow every piece of electronic communication that your daughter makes. If you choose to do this, get help from family and friends because you will go crazy trying to follow every Facebook page, tweet, etc.
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The problem with Facebook is you can create any number of different accounts, so I'd stay away until until she is thirteen and then she will be able to tackle it sensibly.
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Get her to block him in Privacy Settings. He will then be unable to contact her or see any of her posts. Of course she can reverse it if she so wishes, so she would have to be agreeable to this.